A metal is known to show the phenomenon known as transformation (in a broad sense) in which its properties change with temperature, even in the same solid state, and has conventionally been thermally treated by the method involving heating/cooling cycles for the purpose of improvement of its strength, or the like. In case of an alloy composed of two or more types of metals, in particular, each component has its own solubility changing with temperature. Therefore, it is possible to greatly change its properties by changing the quantity of one metal dissolved in another metal by a heat treatment.
For example, an aluminum alloy (hereinafter sometimes referred to as Al alloy), which is relatively low in cost and can be easily utilized among light alloys, has been extensively used for the areas where a reduction in material weight is required (e.g., aircraft and automobiles). An aluminum alloy can have changed mechanical characteristics, e.g., tensile strength and elongation, when subjected to heating and cooling. This is because an aluminum alloy is composed of aluminum incorporated with copper, magnesium, silicon, zinc or the like, and the changes in characteristics are realized by dissolving these elements in the matrix by heat treatment, which is followed by cooling the alloy with water and age-hardening.
More specifically, one of the aluminum alloys for cast and expanded materials is an Al—Cu-based alloy which contains copper, shows a higher strength, and has been extensively used for automobile members around wheels; and in this Al—Cu-based alloy, it is possible to change its mechanical properties by changing the quantity of copper dissolved in aluminum.
An Al—Cu-based alloy is known to dissolve copper to a limited extent at room temperature, and is in the α-phase region at a high temperature. When an Al—Cu-based alloy is heated at a high temperature, therefore, it has the α-phase with copper dissolved in aluminum. The heat-treated alloy will have significantly different properties depending on whether it is rapidly quenched with water or slowly cooled, because of the θ-phase, with deposited aluminum and copper compounds which determine alloy hardness, appearing differently. When quenched, the alloy will have no θ-phase depositing out, but become the supersaturated solid solution which dissolves the same quantity of copper as it is at a high temperature. This treatment is known as solution treatment.
The supersaturated solid solution is unstable, turning stable when exposed to a higher temperature or left at room temperature for extended periods, after the θ-phase emerges. This phenomenon is known as the age-hardening, and the treatment for causing the age-hardening is referred to as the age-hardening treatment. Normally, an artificial age-hardening treatment is conducted to cause the age-hardening treatment by increasing temperature. (The artificial age-hardening treatment is hereinafter referred to merely as age-hardening treatment.) The artificial age-hardening treatment is adopted in order to reduce the treatment time. At the same time, it can generally give better properties, e.g., tensile strength, with the age-hardening treatment at a certain high temperature than the natural age-hardening treatment in which the work is left at room temperature for extended periods.
The solution/age-hardening treatment is an effective heat treatment method for improving mechanical strength of a metallic product.
However, some metallic products are required to have an area having different mechanical properties from the other area, e.g., one part is required to be hardened or to be more ductile than the other. To meet such requirements needs a more complex heat treatment process, accompanied by increased cost. Therefore, such a metallic product is normally heat-treated at temperature set at a level not harmful to any required mechanical property at any place.
For example, an aluminum wheel 20 shown in FIG. 2, an outer rim 21 and a spoke 22 need to have a high strength, whereas an inner rim 23 needs to have a high ductility in addition to high strength. Since it is difficult to partly change heat treatment conditions in the heat treatment with the conventional atmosphere furnace, the whole aluminum wheel 20 is frequently heat-treated under the conditions normally set to improve strength as the major objective with keeping ductility above a certain level.
Therefore, there have been great demands for the heat treatment unit and method which can change heat treatment conditions depending on areas of a metallic product and thereby impart different mechanical properties to each area.
The present invention has been made in view of the above conventional problems. It is an object of the present invention to provide a heat treatment furnace which is improved over the conventional one in that it can give preferable mechanical properties which a specific area of metallic work requires without increasing an investment cost, a heat treatment unit incorporating the same furnace, and a method of heat treatment using the same heat treatment unit. A metallic product having more desired properties can be made thinner to reduce the production cost. In particular for a product of aluminum alloy, which is frequently used to reduce weight, the thinner product is lighter and should contribute to its increased demands.